Current polling by the Cook Political Report indicates that it is likely the Democrats will retain control of the Senate, and may even gain a few seats.   At this point, however, it doesn’t look like they will reach the 60 seats needed for a supermajority.

60 votes are needed in the Senate to end debate or filibuster on a bill via a cloture vote.   (From senate.gov:  Cloture is the only procedure by which the Senate can vote to place a time limit on consideration of a bill or other matter, and thereby overcome a filibuster).    Therefore, a party controlling 60 seats (and who can get a party-line cloture vote) has dominant control of the legislative agenda, as it can bring any issue to the full Senate for a vote.